The effects of thiols, ascorbic acid and thermal stress on the basal (steady-state) levels of oxidative DNA base modifications were studied. In various types of untreated cultured mammalian cells, the levels of total glutathione were found to be inversely correlated with the levels of DNA base modifications sensitive to the repair endonuclease Fpg protein, which include 8-hydroxyguanine (8-oxoG). A depletion of glutathione by treatment with buthionine sulphoximine increased the steady-state level in AS52 Chinese hamster cells by~50%. However, additional thiols in the culture medium did not reduce the level of Fpgsensitive base modifications: 0-10 mM N-acetylcysteine had no effect, whereas cysteine ethylester even increased the oxidative DNA damage at concentrations >0.1 mM. Similarly, ascorbic acid (0-20 mM) failed to reduce the steadystate levels. When AS52 cells were grown at elevated temperature (41°C), the steady-state level of the oxidative DNA modifications increased by 40%, in spite of a concomitant 1.6-fold increase of the cellular level of total glutathione. Depletion of glutathione at 41°C nearly doubled the already elevated level of oxidative damage. A constitutive expression of the heat-shock protein Hsp27 in L929 mouse fibrosarcoma cells at 37°C increased the glutathione level by 60%, but had little effect on the level of oxidative DNA damage.

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